In the boiler flue, metallurgical high temperature flue gas pipeline and chemical tail gas treatment system of power plant, expansion joint is the key component to absorb heat displacement, isolate vibration and protect equipment safety. However, no matter how high-quality expansion joint products are, if the installation link is not up to standard, its service life will be greatly shortened, and even cause flue gas leakage, environmental protection exceeding the standard or pipeline structure damage. Flue expansion joint installation quality requirements cover the whole process from unpacking inspection to final acceptance, and negligence in any one link may lead to serious consequences. According to relevant construction specifications and engineering practice, this paper systematically sorts out the core quality standards, inspection methods and avoidance strategies of common installation defects of flue expansion joint installation, and provides an operable technical guide for construction personnel and quality supervisor.

Quality acceptance requirements before installation

The first step in quality requirements for flue expansion joint installation begins with product admission acceptance. The installation unit shall jointly unpack and inspect with the supervisor and the supplier, and record the following contents:

• Appearance inspection: The surface of the expansion joint shall be free of mechanical damage, scratches, pits or rust; Non-metallic skin should be free of delamination, bubbles and cracks; The wave pitch of the metal bellows shall be uniform, without twisting or extrusion deformation.
• Dimension check: The measured external dimensions of the expansion joint (length, outer diameter of flange, center circle diameter of bolt hole and hole spacing) shall be consistent with the design drawings, and the allowable deviation shall be: length ±5mm, hole spacing of flange bolt ±1mm.
• Check the nameplate with the documents: the model, specification, design pressure, design temperature and compensation amount on the nameplate should be consistent with the design documents; The accompanying technical documents (certificate of conformity, installation instructions) shall be complete.
• Inspection of transportation fixtures: The temporary transportation fixtures shall be intact and not loose, and shall not be removed in advance before the installation is completed.

If any of the above items are found to be unqualified, it shall be marked and isolated immediately and notified in writing to the supplier for handling. It is strictly prohibited to install the unqualified products in place.

Key Quality Requirements for Installation Process

After entering the installation link, the installation quality requirements of flue expansion joints are further refined into multiple control points:

1. Installation direction and position confirmation

• For an expansion joint with a guide tube, the direction of the guide tube must be exactly the same as the direction of flue gas flow-the arrow indicates downstream.
• The installation position of the expansion joint must conform to the design drawing, and the offset shall not exceed ±10mm.
• For expansion joints with cold tightening (pre-tension or pre-compression) requirements, cold tightening must be carried out according to the design specified value, with an allowable deviation of ±5mm.

2. Flue centering accuracy control

Before installation, the relative position deviation of the flue flanges on the adjacent two sides should be measured. For metal expansion joints, the flatness deviation of flanges on both sides is ≤3mm, and the coaxiality deviation is ≤5mm; For non-metallic expansion joints, the allowable deviation can be relaxed appropriately, but it still needs to be controlled within 50% of the design compensation amount. When the deviation exceeds the limit, the flue support should be adjusted or the transition section should be added first. It is strictly prohibited to forcibly pull the expansion joint in place.

3. Flange connection quality

• Bolt tightening: Bolts of the material specified in the design must be used (304 or 316 stainless steel is recommended for the smoke side). The tightening should be performed symmetrically from the middle to the sides in three times: the first preload to 30% of the target torque, the second to 70%, and the third to 100%. Target torque value reference: 40-50N·m for M12 and 70-90N·m for M16.
• Installation of gasket: If a gasket is used in the design, the gasket should be creased and damaged, and the alignment should be good during installation. For non-metallic expansion joints, the flange surface shall be coated with acid-resistant sealant according to the requirements of the manufacturer, and the glue layer shall be uniform and continuous.
• Skin flatness (non-metallic expansion joints only): Skin tension should be moderate, allowing a relaxation margin of 2%-4%, and no obvious wrinkles or distortions on the surface.

4. Inspection of deflectors and internal components

The deflector is a critical component to protect the expansion joint from high-speed smoke flushing. After installation, confirm that:

• The deflector is in the correct direction, and the overlap direction is along the flue gas flow direction.
• The clearance between the deflector and the bellows or the inside of the skin conforms to the design (usually 10-20mm), and must not be touched or rubbed.
• The deflector weld is full and crack-free.

5. Setting of fixing and guiding brackets

• The fixed brackets on both sides of the expansion joint must be installed and reach the design strength after the expansion joint is in place and before the hydraulic test.
• The position and spacing of the guide brackets shall conform to the design drawings, and ensure that the displacement direction of the pipeline is consistent with the design compensation direction of the expansion joint.
• It is strictly prohibited to use the expansion joint itself as a lifting point or temporary support point during pipeline installation.

Inspection and acceptance criteria after installation

The last barrier of the installation quality requirements of flue expansion joints is the inspection and acceptance after installation, which mainly includes the following three items:

1. Appearance and dimension review

There shall be no visible damage to the expansion joint and connecting flange; All the bolts are tightened in place, exposing 2-3 screw teeth of the nut; Transport fixtures have been completely removed; The deviation between the size of the cold position of the expansion joint and the design value shall not exceed ±10mm.

2. Airtightness test

Air tightness test shall be performed after system recovery. The test pressure is 1.05-1.10 times the design pressure, but the maximum is not more than 50kPa (non-metallic expansion joint). Apply soapy water to paint all flange joints and bellows/skin surfaces, and hold pressure for 10 minutes without air bubbles.

Special Note: It is strictly prohibited to use high-pressure gas (> 0.1MPa) to directly test the pressure of non-metallic expansion joints to avoid skin bursting.

3. Hot state review

After the system heats up to a stable operating temperature for the first time, a thermal inspection shall be arranged:

• Measure the actual displacement of the expansion joint and compare it with the design value. If the deviation exceeds 15%, the cause should be analyzed and dealt with.
• Heat tighten the flange bolts to achieve the specified torque.
• Check the skin or bellows for abnormal deformation, leakage and whistling sound.

Four Common Installation Defects and Avoidance

In engineering practice, typical defects resulting from violations of quality requirements for flue expansion joint installation include:

epilogue

The installation quality requirements of flue expansion joints run through every link of product acceptance, flue alignment, flange connection, deflector inspection and airtightness test. Strictly follow the standardized installation process so that the expansion joint can operate reliably within the design life; Conversely, negligence at any one step can lead to leaks, breakages and even safety accidents. For equipment managers in power plants, steel and chemical industries, establishing a set of quality control lists covering the whole installation process is the basis for ensuring the long-term stable operation of flue systems.